1. Field of the Invention
The present invention relates to an image forming apparatus, and particularly to a structure of a fixing device contained in the image forming apparatus.
2. Description of the Related Art
In an image forming apparatus of an electrophotographic type, a photosensitive drum is substantially uniformly charged, and then a laser scanning unit or the like conducts exposure on the photosensitive drum to form an electrostatic latent image according to an image signal. Then, toner that is charged by a developer is supplied onto the photosensitive drum to visualize a toner image, which is transferred onto a recording paper such as a transfer paper sheet. The toner image transferred onto the recording paper is merely born on the recording paper, and is not fixed thereto. Therefore, a fixing unit arranged in the image forming apparatus applies heat and pressure for thermally welding and fixing it so that a fixed image is formed on the recording paper.
Japanese Laid-Open Patent Publication No. 2006-133318 has disclosed a structure in which a heating member containing a halogen lamp, a pressing member pressed against the heating member and a thermister sensing a temperature of the heating member are arranged in a casing as basic components of a fixing unit, respectively. In this fixing unit, the toner image that passes through a nip portion formed by the heating and pressing members pressed together is heated by heat of the heating member that is heated by heat radiated from the halogen lamp, and receives a pressure from the heating and pressing members pressed against it so that the toner image is fixed onto the recording paper. The casing is provided with an entry port for transporting the recording paper thereinto and an exit port for discharging the recording paper therefrom. The recording paper bearing the toner image is transported into the casing through the entry port, and the recording paper bearing the fixed image is transported from the casing through the exit port.
Conventionally, a configuration for preventing diffusion of heat generated by a fixing device and improving a heat retaining effect has been employed. For example, Japanese Laid-Open Patent Publication No. 2006-133318 has disclosed a structure in which openable shutter members are arranged in a sheet entry port and a sheet exit port, respectively. Japanese Laid-Open Patent Publication No. 07-064422 has disclosed a structure in which an openable shutter is arranged in a heat insulator covering a fixing roller and a pressing roller. When printing is not performed, the shutter closes to prevent external diffusion of heat from a fixing device. In a printing operation, the shutter opens to allow passing of a paper sheet.
According to the above disclosed fixing device, however, the shutter must open in the sheet transporting operation, and the heat insulation can be performed in the non-printing operation. However, in the printing operation, i.e., in the sheet transporting operation, the heat insulation cannot be performed, resulting in a problem that the heat escapes from the inside of the fixing device due to an air flow formed on a sheet surface.